1. Technical Field
The present invention generally relates to 1-bit audio signal-processing technology, and more specifically to offset cancellation in an electronic device using said technology, such as for instance a 1-bit Digital-to-Analog Converter (DAC).
It finds application, for instance, in Class-D or class AB audio driver circuits with, e.g., Pulse Density Modulation (PDM) digital inputs, for wireless devices with an audio system, such as mobile phones, smart phones, digital audio players, PDAs, digital tablets, or other similar wireless products.
2. Related Art
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Audio signals are stored in digital form (for example MP3s) and, in order to be heard through speakers or a headset, they must be converted into an analog signal. DACs are therefore found in almost all modern wireless devices with an audio system.
Most current digital audio systems use multi-bit Pulse Code Modulation (PCM) to represent the audio signal in the digital domain. PCM has the advantage of being easy to manipulate. It allows signal processing operations to be performed on the audio stream, such as mixing, filtering, and equalization.
One-bit audio signal representation is another way of conveying audio digitally, in mono or stereo, over a clock/data pair. This scheme is also called Pulse Density Modulation (PDM) because the original analog waveform is converted to a density of pulses. The resulting pulses are digital, and are therefore resistant to distortion and noise which may result from signal-processing techniques applied to it.
Because it uses only one bit to convey audio, PDM is simpler in concept and execution than PCM. Recently, PDM has become very popular in the design of digital audio systems, e.g. as a way to deliver audio from microphones to the signal processor in mobile phones. PDM is ideally suited for this task because it brings the benefits of digital signal processing, such as low noise and freedom from interfering signals, at low cost.
A 1-bit audio driver circuit is basically the combination of a 1-bit DAC followed by an analog filtering amplifier. It may be used as an analog front-end for driving the load of the audio system, for instance a headphone or speakers, within a digital audio system using PDM.
Basically, however, the DAC exhibits a natural offset, which takes the form of a DC voltage applied to the load. This offset causes a DC current to flow through the headphone or speakers, which may result in a reduction of their life time. In addition, the DAC offset is responsible for an unpleasant “CLICK” or “POP” being audible to a user upon the on-switching of the audio system.
Absent any offset cancellation, the offset can provide a lot of “CLICKs” or “POPs” to the headset or speaker at the start-up, particularly with a high gain output amplifier. It also reduces the life time of these transducers.
Assuming a digital/analog partitioning including Base-band digital part and analog front-end within the same chip, it is relatively easy to cancel at the digital part the natural offset provided by the analog front-end. Indeed, the offset at the output can be measured once, for instance at the time of industrial test, and saved in a register, e.g. a One Time Programmable (OTP) memory register, in the Base-band digital part in order to remove it by digital signal processing. Such offset cancellation scheme can advantageously have the same granularity (quantization) as the digital data input.
However, with a partitioning including digital Base-band in a part, for example using a PDM architecture, and analog front-end in another part, it may be more difficult to remove the natural offset of the DAC. Cancelling the offset by subtracting a fixed value from the audio data at digital level is not really an option, due to the response delay within the digital chain. The difficulty for offset cancellation at analog level stems from the poor resolution of the DAC used in such audio systems relying on the over-sampled, noise-shaped principle tied to the 1-bit audio signal-processing technology.
Even if it was a 5 bits partitioning, with a half Full-scale (½ FS) providing 1 Vrms, the maximum granularity for residual offset after cancellation would be +/−FS/16, namely +/−88 mV. Now, with any headset or speaker sensibility, the “POP” provided by 88 mV step relatively to 1 Vrms is very high. Indeed, with 1 Vrms analog audio signal from the DAC, followed by a 14 dB gain amplifier in order to provide 5 Vrms at the output, a 8 ohms loudspeaker with 91.5 dB SPL/W at 1 meter provide 95.5 dB SPL. With the same set-up, a +/−88 mV×5 (i.e., 14 dB) step at power-up provide 74 dB SPL on 1 msec, which generates an audible “POP”.